This invention relates to ring lasers capable of supporting running waves in multidirection of operation, and in particular, relates to a method and apparatus for operating ring lasers in single mode, frequency and unidirection of propagation. The present invention finds particular application to dye lasers of the type in which a laser is used to pump a liquid dye contained within a resonator cavity.
In a normal dye laser the amount of single frequency power available is limited by spatial hole burning. Typically, several standing wave modes of resonance of a laser light beam are established and form nodes and anti-nodes within the gain medium and new modes appear at the places where the original mode had a node of the electric field. These detract from the power output capability of the laser in single frequency operation. An improved form of dye laser takes the form of a ring laser which consists of a plurality of laser mirror elements disposed to support laser oscillation in both the forward and backward mode in the form of running waves so that ring lasers are capable of maintaining a greater portion of the gain medium in single frequency oscillation and therefore are capable of higher power outputs by eliminating spatial hole burning. The running waves in ring lasers are counter-rotating and termed herein forward and backward waves, i.e., clockwise and counterclockwise which can be shown to be necessary to obtain single frequency operation. However, in order to obtain maximum, single frequency output of such a device, it is desired that one of the running waves predominate and that the output be taken from the predominant wave.
Attempts to achieve predominance of one wave over the other have not been completely successful and therefore unidirectional stable operation of such ring lasers has not been achieved in a satisfactory manner. In the ring laser a plurality of mirrors are utilized, three being the theoretical minimum but four being typical in order to more conveniently arrange the laser along a linear path. The laser beam passes clockwise (forward) or counterclockwise (backward) between the mirrors. If an extra mirror is placed behind one of the ring cavity mirrors which is allowed to have a transmission characteristic, selective retroreflection of radiation from behind the one mirror may be arranged and this procedure would appear to make the ring laser operate in a single direction. However, it appears that it is very difficult to simultaneously obtain single frequency operation of the ring laser by that procedure. There is, therefore, a need for a new and improved method and apparatus for controlling ring lasers which will provide for single mode, single frequency, and single direction of operation of uniform, high output.